This is a national stage of PCT/DE99/00932 filed Mar. 27, 1999 and is based upon the German national application 198 14 748.1 filed Apr. 2, 1998 under the International Convention.
The invention relates to a method for alloying steel by introducing metallic additives and/or reducing agents in powder form into a fluid metallic melt. The invention relates further to a device for implementing the method.
The present invention pertains to the field of secondary metallurgy, i.e. the further processing of, for instance, a steel produced by means of a refining process in a converter or in an electric-arc oven. A particular part of this further processing is establishing the composition of the steel alloys, during which the desired metallic additives are introduced. Optionally in this phase also an additional deoxidation, decarburization or desulfurization of trace elements can be performed.
From DE 42 37 177 A1 a mechanically adjustable dosage and delivery device for goods in powder form is known, which introduces one or more kinds of various powders in metered amounts into a pig iron melt, through a delivery device ending in an immersion lance. This dosage and delivery device is used for blowing and particularly for co-injection of calcium carbide and magnesium powders into pig iron melts.
The DE 44 00 029 A1 attempts in a similar fashion to achieve an adjustably metered introduction of additives into electric-arc furnaces, crucibles or pouring ladles. In all these cases the respective solid material in powder form is introduced by means of a carrier gas which according to the state of the art amounts to approximately 80% and more of the total gas-solids mixture. However, a difference in flow rates or flow velocity of the used carrier gases and the solid materials makes impossible a precise dosage or adjustment of the alloying elements to be introduced, or of the alloying elements in relation to the deoxidation elements, which negatively influences the production of the alloyed steel.
For this reason in establishing the composition of the steel alloy, one works with hollow wire in whose inner space the metals and/or deoxidizing agents are arranged in compressed powder form. This wire is unwound by means of a spooler directly into the bath, where the wire melts and releases the alloying elements. For instance wires with a diameter of up to 23 mm and a wall thickness of 0.5 mm are used and the filling material is disposed therein. The production of such wires filled with alloying elements in powder form is expensive. Furthermore the wire spooling method has the disadvantage that the melting wire end does not always lie in the desired location under the bath surface, so that different concentrations of alloying elements arise in the melt.
The more cost-efficient and technically simpler technology, which allows the alloy powders to trickle from above onto the bath surface has again the disadvantage that, because of the existing bath movements, which are directed radially outward on the surface of the metal bath, the alloying agents are pushed against the ladle or converter walls, where they adhere to the slag deposits or to the inner surface of the melt container. Since this happens in an uncontrollable manner depending on different bath movements, this method also can lead to undesirable alloy compositions, which do not correspond with the metal alloying requirements.
It is therefore the object of the present invention to so improve the method described at the outset that alloying additives alone or alloying additives in connection with reducing agents can be introduced in a precise dosage into the melt, with better dosage accuracy.
This object is achieved, according to the invention, in that the alloying additives and/or the alloying additives and reducing agents are introduced in the melt as powders (dry) from a supply container via a conveying device directly to one or more lances and/or below-bath nozzles into the melt, or are introduced into the melt in a mixture with a fluidizing agent, whose proportion in the injected amount is xe2x89xa620%, whereby the pressure used for the powder conveyance is at least 20 bar, preferably 40 bar, or whereby the powders in the delivery line are discharged by means of a plunger covering the cross section of the delivery line.
The injection technology of solids via lances or below-bath nozzles is used in a modified manner, in that the respective additives are injected into the melt dry (i.e. without gas or other fluidizing agent) at a conveying pressure of 40 bar and more or that the amount of the fluidizing agent is limited to a maximum of ⅕ in the gas-solid powder mixture and the mixture is injected at a pressure of more than 20 bar. The conveying pressure decreases correspondingly to the friction losses towards the lance or below-bath nozzle, whereby the pressure at which the powders are discharged decreases to values which are only slightly above the ferrostatic pressure. Surprisingly it has been found that an optimal dosage can be achieved, in spite of the considerable friction losses, which exist and which according to earlier assumptions were considered to endanger a uniform introduction of additives. It is also surprising that, when one is working with a lower proportion of fluidizing agents or without fluidizing agents, pressures between 20 bar and 40 bar are sufficient in order to overcome the frictional resistance.
As an alternative it is possible to push out the powder in the delivery line and/or the lance by means of a plunger covering the entire cross section of the delivery line. This method limits the amount of powder which can be introduced by means of the plunger, as well as the cross section of the delivery line through which the plunger is moved. The delivery line or lance segment has to be filled again after each powder discharge. Suitably the powder exit point at the lance is closed by a cover during refilling, which melts away when the lance is introduced into the melt.
As already mentioned, fluidizing agents can also be used, such as small amounts of inert gas, particularly argon or nitrogen, or of liquid hydrocarbons, such as heating or diesel oil, heavy oil, waste oil, rape seed oil or paraffin. The use of liquid hydrocarbons, such as waste oil, which burns in the melt, create disposal possibilities, which make it possible to avoid the treatment of waste oil as a special disposal waste.
So for instance preferably powders with a maximum grain size of 1 mm, preferably of maximum 0.1 mm, are used, particularly in the xe2x80x9cdryxe2x80x9d injection without gas or liquids.
A fluidization of the powders can also be performed so that compressed alloying additives form a core surrounded by a shell containing lubricants, e.g. paraffin, wax, oil or the like. The respective particles consisting of core and shell can be used without the addition of further fluidizing agents, or with a further small proportion of gaseous or liquid fluidizing agents.
According to a further embodiment of the invention, the alloying additives are injected in powder form through a swingable lance.
By this one has to understand such oscillating motions which make possible to move the lance exit opening out of the bath, as well as such oscillating motions which allow the setting of different positions of the lance exit opening below the bath surface.
Advantageously the lance is designed as a ring nozzle, and in a further embodiment as a double-wall ring nozzle, whereby through one of the exit openings defined by the double wall inert gases can be injected into the melt, by means of which the lance can be cooled and/or a stirring motion can be produced in the melt. An optimization of this stirring motion can be achieved when the lance exit opening is designed so that the inert gases are delivered tangentially. Through the second exit opening, the dry or fluidized powder, consisting of additives, is then pressed in. In this way it is possible to generate different bath motions with the pure inert gas injected through the lance, depending on the dosage of the additives, which are supposed to be the more intensive the greater the amount of metal or deoxidizing agents introduced per unit time. Different from the methods known to the state of the art, it is possible to adjust independently the dosage amount per time unit and the bath motion.
According to a further development of the invention, a pressure between 50 bar and 100 bar is used for the compression of the additives in powder form.
The aforedescribed process of the invention, which can be applied in steel plants as well as in foundries, offers several possibilities for introducing alloy components, such as aluminum, silicon, magnesia, calcium, and various carbides into the melt. In addition to the volumetric dry transport over a dosage and compression transport, there is the volumetric conveyance of the gas-powder mixture or of the gas-liquid mixture, whereby the proportion of gas or liquid amounts to a maximum of 20% of the mixture. The fourth possibility consists in the aforedescribed volumetric transport of the mixture with a xe2x80x9cdry corexe2x80x9d of the additives to be introduced and a wet shell or a pasteous powder mix, consisting preferably of hydrocarbons, which have lubricant capability.
Thereby the following advantages result:
The proportion of fluidizing agent (inert gas, liquid) is reduced to a minimum. The dosage takes place via a volume measurement, which is more accurate than the conventional gravimetrical method, or of a throughflow measurement. The conveying pressure can be varied depending on the flowability of the powdery solid materials. Finally it is possible to establish the composition of the alloys according to specification during treatment. The dosages can be performed dust-free and with reduced emissions. Since there are no large amounts of transport gas (as is normally the case according to the state of the art), or only a small fluidization in the supplied product, the additives, the residence time of the additives in the reaction zone of the lance opening in the melt also increases.
For implementing the aforedescribed method a device is used wherein, a delivery line having a high-pressure pump leads directly from a supply container to a lance or a below-bath nozzle. These measures reduce considerably the energy consumption with respect to the conventional techniques. Also reduced are the service and maintenance.
According to a further development of the invention, the delivery line has an air or liquid inlet, preferably with an dosage adjusting device depending on the amounts supplied from the supply container, by means of which the desired mixing ratio powder: (fluidizing agents, respectively lubricants) can be set.
The lance has a double shell with two discharge openings, through one of which flows the metal in powder form or the reducing agent, optionally fluidized, and through the other flows the inert gas. In the ideal case, the discharge opening, respectively openings of the lance are arranged so that they abut tangentially. The lance itself is preferably swingable. The bath (the melt) can be arranged in a converter, a ladle or a torpedo carriage.